Moisture stable bias transfer roll

ABSTRACT

Electrostatic transfer of charged particles to a transfer member is accomplished using a roller electrode composed of an electrically conductive core or base having an electrically relaxable primary layer, a self-levelling secondary layer superimposed thereover, said layers being substantially hermetically sealed by a third overcoat layer. The overcoat layer is composed of a polymeric material having a low water vapor permeability constant such that the lower layers of the roll will be rendered insensitive to abrupt humidity changes and consequent electrical instability.

[22] Filed:

Swift 1 Nov. 18, 1975 MOISTURE STABLE BIAS TRANSFER ROLL [75] Inventor:Joseph A. Swift, Ontario, NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

Sept. 9, 1974 21 Appl. No; 504,471

[52 U.S.Cl 355/3 R; 96/1.4;1l8/62 l [51] Int. Cl. GOSG 15/16 [58] Fieldof Search 355/3 R; 96/1.4; 118/621 [56] References Cited UNITED STATESPATENTS 5/1973 Camis et al., 355/3 R X Sullivan 96/1.4 X

Primary Examiner-Robert P. Greiner [57] ABSTRACT Electrostatic transferof charged particles to a transfer member is accomplished using a rollerelectrode composed of an electrically conductive core or base having anelectrically relaxable primary layer, a selflevelling secondary layersuperimposed thereover, said layers being substantially hermeticallysealed by a third overcoat layer. The overcoat layer is composed of apolymeric material having a low water vapor permeability constant suchthat the lower layers of the roll will be rendered insensitive to abrupthumidity changes and consequent electrical instability.

7 Claims, 3 Drawing Figures V01 (/ME RES r/v/rr (0/0; an)

US. Patent Nov. 18, 1975 7 Sheet 1 of 2 MOISTURE STABLE BIAS TRANSFERROLL BACKGROUND OF THE INVENTION This invention relates toelectrophotography, in particular, to an electrically stable apparatusfor transferring toner images from one surface to another.

In conventional electrophotographic xerography, a photosensitive plate,which consists of a photocont active coating placed over conductivebacking, is charged uniformly and the charge plate then exposed to alight image of an original. Under the influence of the light image, thecharge on the plate is selectively dissipated to record the originalinput scene informa tion on the plate in the form of a latentelectrostatic image. The latent image is developed, or made visible, byapplying oppositely charged toner particles to the plate surface in amanner so that the toner particles are attracted into the imaged areas.The developed images In corona induced transfer as, for example,disclosed in U.S. Pat. No. 2,836,725, the final support sheet is placedindirect contact with the toner image while the image is supported onthe photoconductive surface. The back of the sheet, that is, the sideaway from the image, is sprayed with a corona discharge having apolarity opposite to that carried by the toner particle causing thetoner to be electrostatically transferred to the sheet. Biased rolltransfer has been tried with some limited success as a means ofcontrolling the forces acting on the toner during transfer. This type oftransfer is disclosed by Fitch in U.S. Pat. No. 2,807,233 and involvesthe use of a metal roll coated with a resilient coating having aresistivity of about to 10 cm. Be-

cause of the resistivity of the coating, the amount of .bias that can beapplied to the roll is limited to relatively low operating valuesbecause, at the higher ranges, the air in and about the transfer zonebegins to break down, i.e., ionizes causing the image to be degradatedduring transfer. Shelffo in U.S. Pat. No.

3,520,604, suggests that the resilient coating have a resistivity ofbetween 10 l 0 ohms cm. Here, in order .to give the roll the neededresiliency required in most practical applications, the coating must berelatively thick. A thick coating of high resistivity acts to build up asurface charge on the roll resulting'in air break down in the transferregion and eventually copy degradation.

More recently, improved bias transfer members have been disclosed whichovercome many of the electrical and image degradation problemsassociated with some previous transfer techniques. U.S. Pat. No.3,702,482 discloses a multiple layer transfer roll member fortransferring xerographic images under controlled con- :ditions. Themember is capable of electrically cooperating with a conductive supportsurface to attract charged toner particles from the support surfacetowards the member or towards a transfer material such as paperpositioned'there between, the member having a conductive substrate forsupporting a biased potential thereon, an intermediate blanket (primarylayer) placed in contact with the substrate to the outer biased toconstant potential (constant voltage) levels.

periphery of the blanket and a relatively thin outer coating (secondarylayer) placed over the blanket having an electrical resistivity tominimize ionization of the atmosphere when the transfer member is placedin electrical cooperation with the image support surface and providing agood toner release property enabling the device to be cleaned of saidtoner. U.S. Pat. No. 3,781,105 discloses a simulator transfer memberemployed in conjunction with a variable electrical bias means forregulating automatically the electrical field levels at various pointson the transfer member during the transfer operation and providingconstant current control.

In the preferred embodiment, the transfer member disclosed in theaforementioned two U.S. patents consists of a roller having a centralbiasable conductive core further having an intermediate blanket orelectrically relaxable layer (primary layer) surrounding and inelectrical contact with the core, and further having a second blanket orelectrically self-levelling outer layer (secondary layer) surroundingand in electrical contact with the primary layer. Under operatingconditions, it is desirable for optimum transfer to maintain arelatively constant current flow of less than about 30 micro amps in thenip area between the transfer roll surface, transfer material, andphotoconductive surface from which a developed image is to betransferred. For this condition to exist at given potentials, theresistivity of the primary and secondary layers must lie within criticalvalues and preferably be relatively constant under normal anticipatedextremes of operating conditions. Preferably, it has been found that theprimary layer should be a resilient elastomeric material having a volumeresistivity within the range of 10 to less than 10 ohm cm, and thesecondary layer should be also a resilient material having a volumeresistivity within the range of 10 to 10 ohm cm.

In practice it has been found that elastomer materials used in thetransfer member such as polyurethanes which exhibit resistivities withinthe above ranges are moisture sensitive such that resistivity may varyby as much as a factor of 50 between 10% and 80% RH. as a function ofthe amount of moisture absorbed from or lost to the surroundingatmosphere. For example, in the case of polyurethane materials which areemployed as the primary layer and which have exceptionally goodelectrical characteristics, the volume resistivity may change from 10ohm cm at low moisture contents, i.e., less than about 0.1% moisture, to10 ohm cm at higher moisture levels, i.e., about 2.5% moisture. Otherpolyurethanes suitable for use as the secondary layer exhibitresistivity variations from about 10 to 10 ohm cm as a function ofincreasing moisture content.

The consequent variations in resistivity will ordinarily give rise toerratic performance of the transfer member from day to day particularlyin terms of transfer efficiency unless compensated for by a concomitantchange in the voltages sufficient to maintain a constant nip current, asdisclosed in U.S. Pat. No. 3,781,105.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide a bias transfer roll for use in a xerographictransfer process which is more electrically stable over extremes ofrelative humidity encountered under operating conditions.

Another object is to maximize image quality and transfer efficiency ineffecting transfer of toner from a 3 photosensitive surface to atransfer surface.

These and other objects of the present invention are accomplished usinga bias transfer member comprising a conductive core, an electricallyrelaxable primary layer laminated thereto, a self-levelling secondarylayer superimposed thereover, and a third overcoat layer whichsubstantially hermetically seals said primary and secondary layers. Theovercoat layer is composed of a polymeric material with a low watervapor permeability constant such that the transmission of moisturetherethrough to said primary and secondary layers is severely retarded.The thickness of the layer should be such that the dielectric breakdownstrength of the material is exceeded under operating transferconditions.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a partialsection of the preferred bias transfer roll of the present invention.

FIG. 2 is a graph showing a typical relationship be tween resistivityand moisture content of an elastomeric polyurethane material.

FIG. 3 is a graph comparing fluctuations in moisture content of transferrolls having no overcoat vs. overcoated rolls.

DETAILED DESCRIPTION OF THE INVENTION The exemplary transfer rollershown in FIG. 1 includes a central conductive core or axle 11, which ispreferably a hollow cylindrical tube of conductive aluminum, anelectrically relaxable primary layer 12, an electrically self-levellingsecondary layer 13 and an overcoat layer 14 composed of a polymericmaterial exhibiting a low water vapor permeability constant. Thicknessesof the various layers shown in the drawing are not necessarily to scalebut merely illustrative.

Polymeric materials suitable for use as the transfer roll overcoat layer14 according to the present invention broadly include those materialswhich exhibit a water vapor permeability lessthan the water vaporpermeability of the materials used in the primary and secondary transferroll layers, e. g., polyurethane. Since the main purpose of the overcoatmaterial is to prevent or minimize moisture transmission from theexternal environment to the moisture sensitive primary and secondarylayers, and since the maximum thickness of the overcoat is limited forelectrical reasons as hereinafter disclosed, polymeric materialsexhibiting very low water vapor perrneabilities are much preferred. Thiswould encompass polymeric films having a water vapor permeabilityconstant (P) of less than about 1 X 10 at about 30C., where P isexpressed by the relationship: P=(ml at STP) (cm)/(cm (sec.) (cm l-Ig).In this equation (ml at STP) refers to ml of H 0 absorbed at StandardTemperature and Pressure, (cm) refers to a given length of film, (sec.)refers to timeand (cm Hg) is pressure. Materials satisfying the abovecriteria include low density polyethylene and other polymers whichexhibit a permeability constant less than low density polyethyleneincluding, but not limited to, high density polyethylene, polypropylene,butyl rubber, ethylene/propylene rubber, certain low moisture permeablepolyamides such as nylons, polyvinylidene chloride, polyvinylidenefluoride, polytrifluorochloroethylene, rubber hydrochloride, copolymersand terpolymers of acrylonitrile with vinyl monomers or with monoolefinsand/or diolefins such as styrene/acrylonitrile resins,acrylonitrile/butadiene/styrene (ABS resins) orstyrene-acrylonitrile/butene resins, and like materials. 7

The amount of moisture transmitted through a given film material is afunction of the thickness of the film, i.e., the thicker the film, theless moisture transmitted. Thus, while relatively thick overcoat filmse. g., greater than about 10 mils) might be desirable, the electricalcharacteristics of such films during transfer would be undesirable. Mostpolymeric materials have a volume resistivity in excess of 10 ohm cm. Athick layer of a highly resistive material would reduce the fieldimposed at the transfer nip thereby rendering the transfer rollsubstantially inoperative during the transfer process unless anextremely high and impractical potential were imposed. Therefore themaximum thickness of an overcoat composed of a polymeric material ofhigh resistivity must be such that the dielectric breakdown strength ofthe overcoat layer may be exceeded under operating transfer conditionsthereby rendering the layer electrically invisible to the process. Wherethe moisture barrier polymeric overcoat layer is of relatively lowvolume resistivity, i.e., less than about 10 ohm-cm, the maximumthickness of the layer should be below about 5 mils, preferably below 2mils. Where the resistivity of the polymeric overcoat is high, i.e.,greater than about 10 ohm-cm, the maximum thickness of the layer shouldbe less than about 1 mil. In most cases, the ideal thickness of a highlyresistive overcoat layer for a proper balance of moisture penneabilityand electrical properties is in the order of about 1/10 of the thicknessof the secondary overcoat layer, or within the range of about 0.2 to 0.3mils or 0.0002-00003 inches.

The primary relaxable layer 12 of FIG. 1 comprises a material thatfunctionally takes a selected time period to transmit a charge from theconductive core 11 to the interface between the relaxable layer 12 andthe selflevelling layer 13 sufficient to restore said interface to aboutthe bias potential applied to the core. This selected time period isthat corresponding to the roller surface speed and nip region width,i.e., roughly greater than the time any point on the transfer roller isin the nip region, and is chosen to be approximately one quarter of theroller revolution time. Functionally, this means that the magnitude ofthe external electric filed increases significantly from the pre-nipentrance toward the post-nip exit, while the field within the relaxablelayer diminishes. Thus, a relaxable layer is one that has an externalvoltage profile which is nonsymmetrical about the transfer nip.

The primary layer is formed of an elastomeric or resilient material suchas polyurethane or silicone rubber having a thickness in the range ofabout 0.20 to 0.30 inches, preferably about 0.25 inch and havingsufficient resiliency to allow the roll to deform when brought in movingcontact with a photoconductive surface which may be in the form of aplate, drum or belt. This insures an extended contact region in whichtoner particles can be transferred between the photoconductive surfaceand transfer material. Preferably the primary layer has a durometerhardness in the range of 15-25, Shore A. Because the primary layershould be capable of responding rapidly to the biasing potential toelectrically impart the charge potential on the core to the outer of theroll surface, it preferably should have a resistivity in the order of 10to 10 ohms cm., with about 2.0 X 10 ohms cm. a particularly desirabletarget.

The secondary self-levelling layer 13 is a leaky insulator. The layer isselected for substantially higher resistive values, which in the presentembodiments means in the order of about to 10 ohms per centimeter, morepreferably in the order of 10 to 10 ohm cm. In addition, theself-levelling layer includes materials, (or is so related to therelaxable layer), such that charges applied to the outer surface of theself-levelling layer will be generally dissipated within one revolutionof the roller. This dissipation of charge is desirable to preventsuppression of the transfer field in the nip.

The secondary layer is also formed of a resilient material preferablyhaving a durometer hardness in the range of about 65-75, Shore D, and apreferred thickness in the range of about 0.0020 to 0.0030 inches,preferably about 00025 inch. It has been found that in order to minimizeionization in the atmosphere and in and about the nip contact region, itis preferred that the secondary layer have a target resistivity of about3.2 X 10 ohms cm. A preferred material for use as the secondary layer isa polyurethane marketed by the duPont Company under the trade nameAdiprene.

The moisture barrier polymeric overcoat layer may be applied to theelastomeric surface of the transfer roll by any suitable process whichwill permit the formation of a relatively thin film having asubstantially uniform thickness throughout. Suitable techniques includeforming a solution of the moisture barrier polymer in a suitable solventand applying the solution to the surface of the transfer roll byspraying or dipping. Altematively, thin films of the moisture barrierpolymer may be laminated directly to the transfer roll surface using asuitable adhesive or using heat shrinking techniques.

The following example illustrates the fabrication of a transfer rollhaving a moisture barrier overcoating comprising polyvinylidenechloride.

EXAMPLE 1 A xerographic transfer roll as disclosed in US. Pat. Nos.3,702,482 or 3,781,105 was provided. The roll consists of an aluminumcore, a primary overcoating layer comprising a polyurethane material(Upjohn 2137-, marketed by the Upjohn Corp.) having a thickness of about0.25 inch and a room temperature volume resistivity of about 5 X 10 ohmscm at a moisture content of about 1.5% (50% RH), and a secondaryovercoating layer comprising a polyurethane material (Adiprene L-3l5,marketed by the duPont Corp.) having a thickness of about 0.0025 inchand a room temperature volume resistivity of about 3 X 10 ohms cm at amoisture content of about 1.0% (50% RH).

A solution of polyvinylidene chloride (PVCl was prepared by dissolving10 parts by weight of PVCl in 225 parts by weight ofmethylisobutyl-ketone. About 0.01 parts of a hydrophobic silicon dioxide(Silanox Cabot Corporation) was added as a levelling agent for the PVClThis solution was then loaded into a laboratory spray gun and sprayeduniformly on the peripheral surface of the secondary overcoat layer ofthe above transfer roll. Spraying was continued until sufiicientsolution had been applied to yield, after drying, a PVCl film having auniform thickness of less than about 0.3 mil. The resistivity of PVCl issuch that the thickness of the PVC] overocat should not exceed about 0.3mils so that the dielectric breakdown strength of the layer may beexceeded during the transfer operation i.e., when about 2500 volts isapplied.

The transfer roll was thoroughly dried to yield a three layer structurewherein the elastomeric layers are hermetically sealed by a PVCloverocat having a substantially uniform thickness of about 0.2 mil.

The stability of the transfer rolls of the present invention towardsmoisture transmission and consequent electrical stability isdemonstrated by FIG; 3. A coated roll as prepared in Example 1 wasevaluated vs. a roll with no moisture barrier coating over a period of28 days at various humidity values from RH to 10% RH. As shown in FIG.3, the fluctuations in moisture content (both gain and loss of moisture)were severe with the uncoated roll but relatively stable with the coatedsealed roll. The resistivity of the uncoated roll fluctuates by largeorder of magnitude over the moisture range encountered during the test.Variations in resistivity for the uncoated roll are shown to fluctuatefrom 9 X 10 to 3 X 10 ohm-cm. under low and high humidity conditions,while the sealed roll variation is only 1 X l0 to 9 X 10 ohm-cm.

In practice it has been found that the efficiency of transfer of tonerto the transfer substrate (paper) is maximized at nip currents withinthe range of about 15 to 20 micro amps. Improved stabilization of theresistivities of the primary and secondary overcoat layers of thetransfer roll according to the present invention thus allows forreasonable maintenance of this nip current under operating conditionswithout the need to resort to sophisticated means for altering thevoltage applied to the transfer roll to compensate for largefluctuations in roll resistivity due to changes in humidity conditions.

The transfer roll encompassed by the present invention may be used inconjunction with any suitable electrophotographic apparatus as a meansfor transferring toner particles bearing an electrostatic charge fromthe surface of photoconductive insulating layer to a transfer surfacesuch as paper. Transfer is accomplished as in the prior art by feeding asheet of transfer material into the nip region formed by the surface ofthe transfer roll and surface of a photoconductive insulating materialbearing a developed image, and imposing a potential on the transfer rollsufficient to cause the transfer of the toner material from the surfaceof the photoconductive insulating material to the adjacent surface ofthe transfer material. In practice, any source of electrical powerconnected to the central conductive core of the transfer roll andcapable of placing the transfer roll member at potential sufficient toattract toner images from the photoconductive insulating surface towardsthe roll may be employed. A more complete discussion of the principlesand configurations involved in bias roll transfer may be found in US.Pat. Nos. 2,951,443, 3,620,616, 3,633,543, or 3,781,105.

What I claim is:

1. A transfer member for electrically cooperating with a conductivesupport surface to electrically attract charge particles from thesupport surface towards the member including:

a. a conductive support;

b. a primary elastomeric intermediate layer overlying said conductivesupport, said primary layer having a relatively constant volumeresistivity within the range of about 10 to less than 10 ohms cm;

0. a secondary elastomeric intermediate layer overlying said primarylayer, said secondary layer having a relatively constant volumeresistivity within the range of about 10 to 10 ohms cm; and

7 d. a polymeric overcoating layer overlying and hermetically sealingsaid layers, said overcoating layer having a substantially uniformthickness of less than about mils and a moisture permeability content ofless than about 1 X at 30C.

2. The transfer member of claim 1 wherein said polymeric overcoatinglayer has a volume resistivity of greater than about 10 ohm-cm. and athickness of less than about 1 mil.

3. The transfer member of claim 2 wherein said primary layer has avolume resistivity within the range of about 10 to 10 ohms-cm. and saidsecondary layer has a volume resistivity within the range of about 10 to10 ohms-cm.

4. The transfer member of claim 3 wherein said primary layer has avolume resistivity of about 2 X 10 ohms-cm. and said secondary layer hasa volume resistivity of about 3.2 X 10 ohms-cm.

ides, polyvinylidene chloride, polyvinylidene fluoride,

polytrifiuorochloroethylene, rubber hydrochloride, and copolymers ofacrylonitrile with vinyl monomers, monoolefins or diolefins.

7. The transfer member of claim 6 wherein the polymeric overcoatinglayer comprises polyvinylidene chloride.

1. A transfer member for electrically cooperating with a conductivesupport surface to electrically attract charge particles from thesupport surface towards the member including: a. a conductive support;b. a primary elastomeric intermediate layer overlying said conductivesupport, said primary layer having a relatively constant volumeresistivity within the range of about 107 to less than 1011 ohms cm; c.a secondary elastomeric intermediate layer overlying said primary layer,said secondary layer having a relatively constant volume resistivitywithin the range of about 1011 to 1015 ohms cm; and d. a polymericovercoating layer overlying and hermetically sealing said layers, saidovercoating layer having a substantially uniform thickness of less thanabout 5 mils and a moisture permeability content of less than about 1 X10 8 at 30*C.
 2. The transfer member of claim 1 wherein said polymericovercoating layer has a volume resistivity of greater than about 1014ohm-cm. and a thickness of less than about 1 mil.
 3. The transfer memberof claim 2 wherein said primary layer has a volume resistivity withinthe range of about 109 to 1010 ohms-cm. and said secondary layer has avolume resistivity within the range of about 1013 to 1015 ohms-cm. 4.The transfer member of claim 3 wherein said primary layer has a volumeresistivity of about 2 X 109 ohms-cm. and said secondary layer has avolume resistivity of about 3.2 X 1014 ohms-cm.
 5. The transfer memberof claim 3 wherein the primary layer has a thickness of about 0.25 inch,the secondary layer has a thickness of about 0.0025 inch, and thepolymeric overcoating layer has a thickness of about 0.0025 inch.
 6. Thetransfer member of claim 3 wherein the polymeric overcoating layer isselected from the group consisting of high or low density polyethylene,polypropylene, butyl rubber, ethylene/propylene rubber, polyamides,polyvinylidene chloride, polyvinylidene fluoride,polytrifluorochloroethylene, rubber hydrochloride, and copolymers ofacrylonitrile with vinyl monomers, monoolefins or diolefins.
 7. Thetransfer member of claim 6 wherein the polymeric overcoating layercomprises polyvinylidene chloride.